SnowGauge

SnowGauge PCB v1.2 (TFmini Plus / TSD20 common board)

• Board: 64 x 50 mm, 2-layer, all through-hole. Bottom layer is a GND pour.
• Mounting: 4x M3 (3.2mm) holes at the corners, 57 x 43 mm center spacing (3.5mm from every edge; hole centers at (3.5, 3.5) / (60.5, 3.5) / (3.5, 46.5) / (60.5, 46.5) from the board's top-left corner).
• Fits the previous prototype enclosure (Takachi BCAP091208G, inner 66 x 96 mm) with the 50mm side across the width.
• Battery terminal J1 exits at the bottom edge; TFmini terminal J2 exits at the right edge.
• Generated programmatically: generate_board.py → SnowGauge.kicad_pcb (KiCad 10). DRC: 0 errors, 0 unconnected.
• One board serves both variants: U2 = NJU7223F50 for TFmini Plus / NJU7223F33 for TSD20 (same pinout).

Ordering (JLCPCB, ~1 week to Japan)

1. Go to jlcpcb.com → "Add gerber file" → upload SnowGauge_v1.2_gerbers.zip
2. Settings: 2 layers, 64x50mm (auto-detected), qty 5 (or 10), 1.6mm, HASL, green — all defaults are fine
3. Shipping: DHL or OCS Express (this is what makes it arrive in ~3-4 days; economy mail takes weeks)

Firmware pin map (matches this board — the single source of truth)

Connectors

• J1 (screw terminal 2P, 5.08mm): battery + / − ("+"/"-" on silk)
• J2 (4 positions at 5.08mm = two interlocked 2P blocks): TFmini, top to bottom RED +5V / BLK GND / GRN TX / WHT RX
• J3 (pin header 1x4): SCL / SDA / GND / 3V3 spare I2C
• A1: 2x low-profile 1x7 female headers (2.54mm) — socket for XIAO nRF52840 Sense

Extra BOM lines (in addition to spec §6)

The consolidated BOM for a whole unit (spec §6 parts + these lines, corrected counts, breadboard consumables, TSD20 variant) is docs/01_parts.md / docs/01_parts.csv.

Safety notes silkscreened on the board

• "NO USB WHILE BATTERY CONNECTED!" — disconnect battery before plugging USB into the XIAO
• C4 (OS-CON) polarity marked; D1 cathode marked "K"

Component orientation (U1, U2, Q1, Q2)

The KiCad TO-220 / TO-251 vertical footprints put the reference designator on the metal-tab side, so the part's marked (printed) face points the other way. With the board held "SnowGauge v1.2" up / J1 down: U1 and Q1 have the tab towards the top edge (print facing the XIAO), U2 and Q2 are rotated 180 deg (tab towards the board centre, print facing the top edge). Assembly instructions with a figure: docs/02_assembly.md B-2 (figure regenerated by docs/assets/figures/gen_component_orientation.py).

v1.2 changes (post-review)

• Battery divider now fed from VBAT_SW (the switched rail) instead of VBAT: the ADC node sits at 0V during sleep instead of floating to ~7V through the nRF52840's clamp diode (abs-max violation found in review). Q3, R6 and the D3 GPIO are deleted. Firmware measures battery by enabling SENSOR_EN and reading A0 (x2).
• USB corridor cleared: U1 and C1 moved to the top-left, D1 lowered — a USB-C cable now reaches the socketed XIAO for flashing.
• TFmini wire-order legend moved where it stays visible after C4 is mounted.
• Terminals changed to 3x 5.08mm 2P blocks; the TFmini pair interlocks into one 4P unit on contiguous 5.08mm positions (J2). Standard Akizuki TB111-class parts drop in.